If the documents were returned to sender, then they are not validly served. If they were dropped through the letterbox and then you popped them back into the mailbox writing onto them “return to sender” then they are still validly served because you had them before you return them.
If however they are simply dropped through the letterbox, or dropped at your feet because you refuse to take them off the person, they are validly served because the physical being of the person on the documents is the same physical being regardless of the description.
You could argue that you are not the same person if it gets to court but it’s not a convincing argument because the court is more likely to be involved with the physical being.
You can refuse to accept service of papers even if they are in the correct name.
I can’t comment on decisions in French courts obviously but in some jurisdictions, defamation is criminal.
Any damages awarded in a French court are generally enforceable in the UK court. There are reciprocal arrangements in place.
Bailiffs can only turn up at your address in the UK if there is a warrant of control in respect of the debt, which has been issued by a UK court.
If the papers were dropped on the floor of your house, because your father opened the door, those papers are validly served. By all means argue that the proceedings are invalid and let the court decide whether the name is ***** ***** or not but, as I have already made the point, all that’s likely to do is delay things while the papers are reissued or he makes an application to change the name.